A vehicle propulsion source may provide a small amount of torque (e.g., creep torque) to a vehicle driveline in the absence of driver demand torque at low vehicle speeds. The small amount of torque may allow the vehicle to move at a low speed (e.g., creep at less than 8 KPH) or may allow a vehicle to hold its position on a road having a mild positive grade. If the vehicle is a hybrid vehicle that includes both an engine and a motor, the motor may provide the creep torque to conserve fuel for the engine.
One way of conserving battery charge during creep conditions is to lock a torque converter clutch. By locking the torque converter clutch, torque from the motor may be transferred to vehicle's wheels with fewer torque converter based losses. The motor may rotate at a speed less than engine idle speed when the torque converter is locked and the vehicle is in a creep mode (e.g., a mode where driver demand torque is less than a threshold, vehicle brakes are not applied, and where vehicle speed is less than a threshold speed). If there is a request to start the engine while the motor is providing torque to the driveline during creep conditions, it may be desirable to start the engine without a driveline torque disturbance or an increase in vehicle speed. However, starting the engine may increase the possibility of producing a driveline torque disturbance at low vehicle speeds where driveline torque disturbances may be more noticeable.
The inventors herein have recognized the above-mentioned disadvantages and have developed a driveline method, comprising: opening a locked torque converter clutch during a creep mode in response to a request to start an engine; and increasing a torque converter impeller speed in response to the request to start the engine.
By opening a locked torque converter and increasing torque converter impeller speed, it may be possible to provide the technical result of reducing driveline torque disturbances while maintaining a vehicle creep speed. For example, opening a locked torque converter allows a torque converter's impeller to rotate at a different speed than the torque converter's turbine. Consequently, vehicle speed may be controlled via applying vehicle brakes, and a driveline disconnect clutch may be closed causing a smaller driveline torque disturbance when torque converter impeller speed is close to engine speed. In this way, opening a locked torque converter and accelerating torque converter impeller speed may be beneficial to reduce driveline torque disturbances during an engine start and improve mechanical coupling of the engine to the driveline.
The present description may provide several advantages. For example, the approach may reduce driveline torque disturbances. Additionally, the approach may provide improved vehicle speed control during a vehicle creep mode. Further, the approach may provide improve engine starting during a vehicle creep mode.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.